PROJECT SUMMARY/ABSTRACT Urea cycle disorders are common inborn errors of liver metabolism. With improved therapies such as nitrogen-scavenging agents to prevent elevated ammonia levels, patients with urea cycle disorders have increased survival. However, even in the absence of hyperammonemia, patients with urea cycle disorders (UCDs) may have chronic liver disease. This chronic liver disease appears to be more common in argininosuccinate lyase deficiency (ASLD) as compared to other urea cycle disorders. The hepatic complications in ASLD range from chronic hepatocellular injury, hepatomegaly, fibrosis, cirrhosis, and possibly hepatocellular carcinoma. The pathogenesis for liver disease in ASLD and other UCDs remains unknown, and thus, there are no specific therapies that target this complication. As a first step towards developing therapeutic strategies targeting liver disease in ASLD and other UCDs, we propose to investigate the biochemical basis of liver disease using a mouse model and newly developed hepatocyte-like cell models for this disorder. Since the enzyme, argininosuccinate lyase, integrates two fundamental pathways (urea cycle and citric acid cycle) in the cell, we hypothesize that energy dysregulation results from citric acid cycle disruption and contributes to liver disease in ASLD. We plan to test this hypothesis by pursuing studies that address the following questions: a) Is there mitochondrial dysfunction in the liver of the ASL-deficient mice? b) Does citric acid cycle dysfunction contribute to energy dysregulation in hepatocyte-like cells derived from patients with ASLD? Overall, our studies will combine in vivo murine studies and studies in a new in vitro model to investigate the link between the urea cycle, citric acid cycle and energy dysregulation. Our studies will enable the identification of potential endpoints for future studies of therapeutic strategies for liver disease in ASLD and possibly other UCDs. Moreover, the hepatocyte-like cells generated in this proposal will be an important resource for performing high-throughput screening for therapeutic targets and for dissecting genotype-phenotype relationships in ASLD. Lastly, on broader terms, these studies have the potential to yield important insights into the role of energy dysregulation and urea cycle dysfunction in more common forms of liver disease.